The present invention pertains to active and passive optical fibers and material systems for cladding such optical fibers, and, in particular, to optical fiber claddings having a liquid component. Such claddings improve the optical performance of optical fibers, especially those which carry substantial optical power.
In an optical fiber, light is guided by total internal reflection (TIR) in a transparent core surrounded by a transparent cladding of lower index of refraction. Under ideal lossless conditions, light can be guided with nearly perfect efficiency. In practice, however, fibers can be affected with a variety of more or less severe losses.
At least three major loss mechanisms that reduce transmission efficiency have been identified. Absorption, resulting from impurities in the core or cladding, causes an exponential reduction in transmission with propagation distance. Rayleigh scattering, caused by localized variations in the refractive index of the core and cladding, appears to result from density variations in the core and cladding materials. Major factors which affect density are the uniformity of the starting materials and lay down parameters. Imperfections in the core/cladding interface, which also may result from material variations and lay down parameters, cause the third well-recognized loss mechanism.
Substantial progress has been made in improving fiber performance by reducing or minimizing these loss mechanisms. And, these improvements have resulted in broadened applications for optical fibers. For example, optical fibers have become attractive as laser oscillators and amplifiers. Such devices combine the excellent properties of laser materials with the high energy confinement available in optical fibers. In particular, single mode fibers have been adapted as optical fiber lasers for a variety of system applications. Such fibers exhibit large energy conversion efficiencies and have excellent coupling properties to single mode transmission fibers and, therefore, have important applications in telecommunications fiber transmission systems and networks.
As with other types of lasers, the performance of optical fiber lasers also is measured in terms of slope efficiency. The ultimate performance of a fiber laser device having a known quantum efficiency, the maximum probability that a photon of a particular wavelength produced by a pump source will be converted to laser light at the wavelength of interest, is related to the slope efficiency. This is, the efficiency with which pump radiation can be absorbed and converted to useful laser light by the active material and, in particular, the active material in the fiber core. Snitzer et al., U.S. Pat. No. 4,815,079, issued on Mar. 21, 1989, incorporated herein by reference, discloses a fiber configuration which promotes efficient coupling of radiation to an active single mode core enclosed within a relatively large multimode cladding which, in turn, is surrounded by a light confining outer cladding.
Minns et al., U.S. Pat. No. 5,024,507, issued Jun. 18, 1991, (Minns '507) and Minns, U.S. patent application Ser. No. 07/720,233 (still pending) filed Jun. 24, 1991 (Minns '233), both of which are incorporated by reference, disclose various photopolymerizable compositions for cladding optical fibers. These materials have a remarkably low index of refraction and have essentially no absorption at 807 nm, a particularly useful property at that wavelength. Also, these materials are easily coated on fiber by means of a draw tower.
When these materials are employed as claddings in active devices such as an optical fiber laser having a duty cycle during which high power output (e.g., 2 watts and above) is produced, the slope efficiency degrades with repetitive use with the cladding of Minns '233. Further, it has been found that scattering losses in the multimode core of the fiber appear to increase. While the exact cause is unclear, it is postulated that the polymer cladding may locally delaminate from the glass multimode core resulting in lossy scattering sites at the core/cladding interface.